Pump - For appointments, date nights, building your supply, expressing milk for mixing with cereal, and building a freezer stash, you'll need a pump. If you can afford it, get a double electric. I'm a big fan of the Purely Yours from Ameda. Ameda has a patent on their tubing system, which is designed so that milk can't get up into the tubes. Is it expensive? Yes, but if you do a little looking around on Ebay or Craigslist, you'll find that they hold their value pretty well. (Translation: If you resell it you'll make back a good portion of your moolah.) I think the best deal out there on a new one is through diapers.com. If you've never ordered from them, you can get a promo code for 10 or 15% off. Oh, and don't buy the more expensive package with the carrying case. That's what I use the bag the formula company sent me home from the hospital with for (bwa ha ha ha!!!).
Breast Pads - For wearing around the house when it doesn't matter if you're lumpy, get some washable cloth pads. For leaving the house, I recommend Lanisoh disposables. They're absorbent and are smooth enough so as not to inspire curious strangers to stare at your chest.
Nipple Butter - I have a lanolin allergy, which led me to seek out lanolin-free Earth Mama Angel Baby Nipple Butter. In nearly a year of doing twice the normal amount of breastfeeding, I've never had a problem with chapping or cracking, so that should tell you that it's miraculous stuff. I've also used it as baby chapstick. At $14ish for a tub, it seems expensive, but that little tub lasts a good while.
Milk Storage Bags - Lanisoh wins again. Their bags are reasonably priced, non-leaky, easy to seal, and easy to label.
Breastfeeding Pillow -Make sure you choose something that lifts Baby close to your boobs without you having to bend over. This will a) save you a lot of back pain and b) free you up to do other stuff like reading this fine blog.
Nursing Bras - Check into Bravado. They make 'em in a nice range of sizes and have some pretty cute non-underwire bras, which is no easy thing to find. They are a bit pricey, but if you check for promo codes or sign up for their e-mails, it's not hard to get 20% off. They're also available on Target's website and diapers.com.
Nursing Tops/Dresses - Nursing tops would be a nice thing to have, but if you find yourself breastfeeding during the summer months, you MUST get yourself a nursing dress. Take a second to think this through... What do you do when it's time to nurse? You lift your shirt up... can't do that with a dress, dear. And every gal has to have a cute sundress. Milk and Expressiva make some adorable ones that no one will be the wiser about.